1. Field of the Invention
The present invention relates to a support structure for electrodes of an electro-chemical machining tool.
2. Description of the Related Art
In electro-chemical machining, machining electrolyte is carried up to the surface being machined by a plurality of glass tubes, which thus form the electrodes of the electro-chemical machining tool. The ends of those tubes are arranged adjacent the article to be machined, and as machining occurs holes corresponding to the tubes are cut into the article. This process is used, for example, in drilling cooling holes in the trailing edge of turbine blades or air flow guide vanes of a gas turbine engine. In the terminology of such engines, blades are in the rotating parts of the engine whilst vanes are non-rotating parts.
It is necessary that the tubes be supported so that they are orientated in the correct direction. In order to achieve this, it is usual for the tubes to be clamped between opposed surfaces of two parts of a guide block. An example of this is shown in U.S. Pat. No. 4,188,522.
In existing support structures of this type, where the tubes are clamped between clamping surfaces of two parts of a guide block, one of the clamping surfaces is normally provided with serrations, the width of which is selected in dependence on the tube diameter, and the pitch between the tubes, so that each tube is held in a 3-point mounting, formed by two sides of a serration and the opposed clamping surface of the other part of the block.
In the support structure shown in U.S. Pat. No. 4,188,522, and indeed in all other such structures known to the applicant, the longitudinal axes of the tubes are parallel. However, it has been realised that this causes a problem when drilling e.g. the trailing edge of a turbine blade. Such a blade has an axial twist along its length, and therefore when the line of parallel tubes are placed adjacent the blade edge, only a limited number of tubes will be accommodated on a chord at the curve of the trailing edge of the blade, and that number will further be limited because only some of the tubes which are accommodated will intersect the internal cooling passages within the blade. In practice, it is found that only about 7 holes can be drilled, based on a normal tube pitch, and since 40 to 50 holes are required along the full length of the blade, it is necessary for the hole cutting process to be carried out in a series of passes of the electro-chemical cutting tool along the length of the blade, with only a small number of holes being cut in each pass. The result is that the drilling of the cooling holes in the trailing edge of the turbine blade or vane is a time consuming process.